This invention relates to a surface acoustic wave (SAW) filter comprising resonators for use in a mobile communication equipment such as a portable telephone.
SAW filters have been in use for portable telephones because they can contribute to providing compact devices requiring no tuning. Among such SAW filters, those comprising resonators are considered to be the simplest in structure and the least costly. FIG. 7 shows an example of such a SAW filter of the single-stage type characterized as having interdigitating finger type (or comb-shaped) input and output electrodes 2 and 3 each having elongated "fingers" (or comb teeth) arranged mutually parallel on a piezoelectric substrate 1 at a constant pitch and sandwiched between reflectors 4 also having parallel fingers (as disclosed, for example, in U.S. Pat. No. 5,300,902). The input electrode 2 has fingers connected to and extending from an input terminal 5 as well as those connected to a grounding terminal 7 and extending therefrom towards the input terminal 5. Likewise, the output electrode 3 has fingers connected to and extending from an output terminal 6 and those connected to another grounding terminal 8 and extending therefrom towards the output terminal 6. The input electrode 2 is surrounded as a whole by the output electrode 3 such that the input terminal 5 is surrounded from three sides by the grounding terminal 8 of the output electrode 3 and the grounding terminal 7 of the input electrode 2 is surrounded from three sides by the output terminal 6. The outermost fingers 2a' of the input electrode 2 which are the closest to the output electrode 3 are connected to its grounding terminal 7 and extend therefrom towards the input terminal 5, and the innermost fingers 3a' of the output electrode 3 which are the closest to the input electrode 2 are connected to its grounding terminal 8 and extend therefrom towards the output terminal 6.
FIG. 8, wherein components which are substantially equivalent to those shown in FIG. 7 are indicated by the same numerals, shows another prior art filter with two stages, as an example, formed by connecting a plurality of single-stage type SAW filters described above with reference to FIG. 7. SAW filters of a two-stage (or multi-stage) type have the advantage that they can provide a higher level of attenuation required when used, for example, in a portable telephone.
For expanding the pass band of a SAW filter of this type, it has also been known to use a 36.degree. rotation Y cut X propagation (Y-X) LiTaO.sub.3 substrate or a 64.degree. Y-X LiNbO.sub.3 substrate as its piezoelectric substrate and to arrange the fingers as shown in FIGS. 9A, 9B and 10, that is, each finger is of width equal to 0.25.lambda. (where .lambda. is the wavelength of the surface acoustic wave generated by the filter) and mutually adjacent pairs of fingers are separated by the same distance (=0.25.lambda.) such that the distance between the center lines of mutually adjacent pairs of them is equal to 0.5.lambda., except between the outermost fingers 2a' of the input electrode 2 and the innermost fingers 3a' of the output electrode 3 are in contact with each other, as better shown in FIG. 9B. Passband characteristics of such a filter (with the outermost fingers 2a' of the input electrode 2 and the innermost fingers 3a' of the output electrode 3 disposed next to and in contact with each other in a side-by-side relationship) are generally as shown in FIGS. 11A and 11B. (They naturally depend on the material of the piezoelectric substrate 1 and the structures of the finger (or comb) type input and output electrodes 2 and 3.) FIGS. 11A and 11B show the insertion loss and the voltage standing-wave ratio (VSWR) of a two-stage SAW filter as shown in FIG. 10, of which specifications are given in Table 1 below.
TABLE 1 ______________________________________ Piezoelectric substrate: 36.degree. Y--X LiTaO.sub.3 substrate Thickness of electrode film: 2700.ANG. Electrode material: Aluminum Number of fingers for reflectors: 200 for each reflector Number of fingers for input 41 electrode 2: Number of fingers for output 27 each electrode 3: Center-to-center distance 2.1 .mu.m between adjacent pairs of fingers of input and output electrodes: Length of the fingers by which 170 .mu.m fingers extending from opposite directions overlap each other: Center-to-center distance between 0.25 - the fingers 2a' and and 3a': Number of filter stages: 2 ______________________________________
FIGS. 11A and 11B show not only that there is a ripple of about 1.2 dB inside the passband but also that the VSWR inside the passband becomes as large as 2.2. For practical applications of SAW filters of the type under consideration, however, it is generally required that the ripples be less than 1.0 dB and the VSWR be less than 2.0. In order to satisfy these requirements, it is necessary to design a filter such that its passband will be narrower. With such a design, it is difficult to provide a SAW filter with a broad passband for a large variety of uses.